docs: def format_metric_values(self, metric_values):
      """Formats metric values - used for logging purpose."""
  
      # Early in training, metric_values may actually be None.
      loss_str = 'N/A'
      accuracy_str = 'N/A'
      try:
        loss_str = 'loss: %.3f' % metric_values[0]
        accuracy_str = 'accuracy: %.3f' % metric_values[1]
      except (TypeError, IndexError):
        pass
  
      return '%s, %s' % (loss_str, accuracy_str)